x86/layout: Correct Xen's idea of its own memory layout

x86/layout: Correct Xen's idea of its own memory layout

c/s b4cd59fe "x86: reorder .data and .init when linking" had an unintended
side effect, where xen_in_range() and the tboot S3 MAC were no longer correct.

In practice, it means that Xen's .data section is excluded from consideration,
which means:
 1) Default IOMMU construction for the hardware domain could create mappings.
 2) .data isn't included in the tboot MAC checked on resume from S3.

Adjust the comments and virtual address anchors used to define the regions.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>